Beneath the earth's surface, oil will ooze through rocks if there is enough space between them, but this oil will not accumulate into large quantities unless something traps it in a particular place. There are a variety of geologic traps, which themselves can be broken into categories:

It is important to remember that oil is not all by itself in some sort of underground cave, but is, instead, contained within solid rock - which has enough room within it to actually soak up (or become soaked in, however you look at it) oil.

Structural Traps

There are three basic forms of a structural trapin petroleum geology:

Anticline Trap

Fault Trap

Salt Dome Trap

The common link between these three is simple: some part of the earth has moved in the past, creating an impedence to oil flow.

Anticline Trap

An anticline is an example of rocks which were previously flat, but have been bent into an arch. Oil that finds its way into a reservoir rock that has been bent into an arch will flow to the crest of the arch, and get stuck (provided, of course, that there is a trap rock above the arch to seal the oil in place).

A cross section of the Earth showing typical Anticline Traps. Reseroir rock that isn't completely filled with oil also contains large amounts of salt water.

Fault Trap

Fault traps are formed by movement of rock along a fault line. In some cases, the reservoir rock has moved opposite a layer of impermeable rock. The impermeable rock thus prevents the oil from escaping. In other cases, the fault itself can be a very effective trap. Clays within the fault zone are smeared as the layers of rock slip past one another. This is known as fault gouge.

A cross section of rock showing a fault trap - in this case, an example of gouge. This is because the reservoir rock on both sides of the fault would be connected, if not for the fault seperating the two. In this example, it is the fault itself that is trapping the oil.

Later we'll look at another example of a fault trap: the thrust fault.

Salt Dome Trap

Salt is a peculiar substance. If you put enough heat and pressure on it, the salt will slowly flow, much like a glacier that slowly but continually moves downhill. Unlike glaciers, salt which is buried kilometers below the surface of the Earth can move upward until it breaks through to the Earth's surface, where it is then dissolved by ground- and rain-water. To get all the way to the Earth's surface, salt has to push aside and break through many layers of rock in its path. This is what ultimately will create the oil trap.

Here we see salt that has moved up through the Earth, punching through and bending rock along the way. Oil can come to rest right up against the salt, which makes salt an effective trap rock. However, many times, the salt chemically changes the rocks next to it in such a way that oil will no longer seep into them. In a sense, it destroys the porosity of a reservoir rock.

Thrust Faults

Thrust faulting occurs when one section of the Earth is pushed up and over another section, and they most often occur in areas where two continental plates are running into one another. However, the photos below show sediments that were deposited by glaciers only 10,000 years ago, and these sediments were then run over by a glacial readvance. When the glacier moved back over the sediments, faulting occured. Can you see the faults below?

Now let's draw the faults and rock horizons in, and show how, if the conditions were right, oil might become trapped in this rock. If the rock between the yellow and red lines were sandstone, and rock beneath the red and above the yellow lines were shale, a trap might be set up in this way:

See how the oil gets trapped by the shale above it, as well as by the fault and the shale to the left of it? Of course, this outcrop is only a couple of meters wide, there really is no oil here, and the layers that we've assigned to the rock are mostly imaginary in this case. But the point is, this is exactly how many structural traps are set up below the Earth's surface. Here, we are lucky enough to see it right in front of our very eyes.

Stratigraphic Traps

A stratigraphic trap accumulates oil due to changes of rock character rather than faulting or folding of the rock. The term "stratigraphy" basically means "the study of the rocks and their variations". One thing stratigraphy has shown us is that many layers of rock change, sometimes over short distances, even within the same rock layer. As an example, it is possible that a layer of rock which is a sandstone at one location is a siltstone or a shale at another location. In between, the rock grades between the two rock types. From the section on reservoir rocks, we learned that sandstones make a good reservoir because of the many pore spaces contained within. On the other hand, shale, made up of clay particles, does NOT make a good reservoir, because it does not contain large pore spaces. Therefore, if oil migrates into the sandstone, it will flow along this rock layer until it hits the low-porosity shale. Voilà, a stratigraphic trap is born!

An example of a stratigraphic trap

The above series of diagrams is an attempt to illustrate a type of stratigraphic trap. In the diagram at the upper left, we see a river that is meandering. As it does so, it deposits sand along its bank. Further away from the river is the floodplain, where broad layers of mud are deposited during a flood. Though they seem fairly constant, rivers actually change course frequently, eventually moving to new locations. Sometimes these new locations are miles away from their former path.

In the diagram at the upper right, we show what happens when a river changes its course. The sand bars that were deposited earlier are now covered by the mud of the new floodplain. These lenses of sand, when looked at from the side many years later (the bottom diagram), become cut off from each other, and are surrounded by the mud of the river's floodplain - which will eventually turn to shale. This makes for a perfect stratigraphic trap.

Combination Traps

As the name implies, a combination trap is where two (or more) trapping mechanisms come together to create the trap. In reality, many successful oil traps are combination traps.